Physics 1501: Lecture 16 Today ’ s Agenda

1. Physics 1501: Lecture 16Today’s Agenda • Announcements • HW#6: Due Friday October 14 • Includes 3 problems from Chap.8 • Topics • Chap.8: Gravity and planetary motion • Kepler’s laws • Energy and escape velocity

2. Gravitationaccording to Sir Isaac Newton • Newton found that amoon/ g= .000278 • and noticed that RE2 / R2= .000273 • This inspired him to propose the Universal Law of Gravitation: |FMm|= GMm / R2 amoon g R RE G = 6.67 x 10 -11 m3 kg-1 s-2

3. Gravity... • The magnitude of the gravitational force F12 exerted on an object having mass m1 by another object having mass m2 a distance R12 away is: • The direction of F12 is attractive, and lies along the line connecting the centers of the masses. m1 m2 F12 F21 R12

4. Lecture 16, Act 1Force and acceleration • Suppose you are standing on a bathroom scale in Physics 203 and it says that your weight is W. What will the same scale say your weight is on the surface of the mysterious Planet X ? • You are told that RX ~ 20 REarth and MX ~ 300 MEarth. (a)0.75W (b)1.5 W(c)2.25 W X E

5. Kepler’s Laws • Much of Sir Isaac’s motivation to deduce the laws of gravity was to explain Kepler’s laws of the motions of the planets about our sun. • Ptolemy, a Greek in Roman times, famously described a model that said all planets and stars orbit about the earth. This was believed for a long time. • Copernicus (1543) said no, the planets orbit in circles about the sun. • Brahe (~1600) measured the motions of all of the known planets and the position of 777 stars (ouch !) • Kepler, his student, tried to organize all of this. He came up with his famous three laws of planetary motion.

6. Kepler’s Laws • After 20 years of work on Tycho Brahe’s data, Kepler formulated his three laws: • 1. All planets move in elliptical orbits with the sun at one focal point. • 2. The radius vector drawn from the sun to a planet sweeps out equal areas in equal times. • 3. The square of the orbital period of any planet is proportional to the cube of the semimajor axis of the elliptical orbit. • It was later shown that all three of these laws are a result of Newton’s laws of gravity and motion.

7. Kepler’s Third Law Let’s start with Newton’s law of gravity and take the special case of a circular orbit. This is pretty good for most planets.

8. Kepler’s Second Law This one is really a statement of conservation of angular momentum.

9. Kepler’s Second Law dA R dR 2. The radius vector drawn from the sun to a planet sweeps out equal areas in equal times.

10. Kepler’s Second Law dA R dR

11. Example • The figure below shows a planet traveling in a clockwise direction on an elliptical path around a star located at one focus of the ellipse. When the planet is at point A, • a. its speed is constant. • b. its speed is increasing. • c. its speed is decreasing. • d. its speed is a maximum. • e. its speed is a minimum. Active Figure

12. Lecture 16, Act 2Satellite Energies • Imagine a comet in an elliptical orbit around the sun such that at the perigee it is a distance r from the sun and at the apogee a distance 2r. What is the ratio of the comet’s speed at perigee versus that at apogee, vp/va. (a)1/4(b)1/2(c)1 (d)2 (e)4 Hint: Use conservation of L

13. Example • A satellite is in a circular orbit about the Earth at an altitude at which air resistance is negligible. Which of the following statements is true? • a. There is only one force acting on the satellite. • b. There are two forces acting on the satellite, and their resultant is zero. • c. There are two forces acting on the satellite, and their resultant is not zero. • d. There are three forces acting on the satellite. • e. None of the preceding statements are correct.

14. U RE r 0 Energy of Planetary Motion A planet, or a satellite, in orbit has some energy associated with that motion. Let’s consider the potential energy due to gravity in general. Define ri as infinity

15. We can solve for v using Newton’s Laws, Plugging in and solving, Energy of a Satellite A planet, or a satellite, also has kinetic energy.

16. Energy of a Satellite So, an orbiting satellite always has negative total energy. A satellite with more energy goes higher, so r gets larger, and E gets larger (less negative). It’s interesting to go back to the solution for v. v is smaller for higher orbits (most of the energy goes into potential energy).

17. Lecture 16, Act 3Satellite Energies • A satellite is in orbit about the earth a distance of 0.5R above the earth’s surface. To change orbit it fires its booster rockets to double its height above the Earth’s surface. By what factor did its total energy change ? (a)1/2(b)3/4(c)4/3 (d)3/2 (e)2

18. Escape Velocity • Normally, if I throw a ball up in the air it will eventually come back down and hit the ground. • What if I throw it REALLY hard ? • Two other options • I put it into orbit. • I throw it and it just moves away forever – i.e. moves away to infinity

19. Orbiting • How fast to make the ball orbit. • I throw the ball horizontal to the ground. • We had an expression for v above,

20. Escape Velocity • What if I want to make the ball just go away from the earth and never come back ? • (This is something like sending a space ship out into space.) • We want to get to infinity, but don’t need any velocity when we get there. • This means ETOT = 0 Why ??

21. Example • Which of the following quantities is conserved for a planet orbiting a star in a circular orbit? Only the planet itself is to be taken as the system; the star is not included. • a. Momentum and energy. • b. Energy and angular momentum. • c. Momentum and angular momentum. • d. Momentum, angular momentum and energy. • e. None of the above.